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Subject[tip: locking/core] seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support
The following commit has been merged into the locking/core branch of tip:

Commit-ID: 8117ab508f9c476e0a10b9db7f4818f784cf3176
Gitweb: https://git.kernel.org/tip/8117ab508f9c476e0a10b9db7f4818f784cf3176
Author: Ahmed S. Darwish <a.darwish@linutronix.de>
AuthorDate: Fri, 04 Sep 2020 17:32:30 +02:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 10 Sep 2020 11:19:31 +02:00

seqlock: seqcount_LOCKNAME_t: Introduce PREEMPT_RT support

Preemption must be disabled before entering a sequence counter write
side critical section. Otherwise the read side section can preempt the
write side section and spin for the entire scheduler tick. If that
reader belongs to a real-time scheduling class, it can spin forever and
the kernel will livelock.

Disabling preemption cannot be done for PREEMPT_RT though: it can lead
to higher latencies, and the write side sections will not be able to
acquire locks which become sleeping locks (e.g. spinlock_t).

To remain preemptible, while avoiding a possible livelock caused by the
reader preempting the writer, use a different technique: let the reader
detect if a seqcount_LOCKNAME_t writer is in progress. If that's the
case, acquire then release the associated LOCKNAME writer serialization
lock. This will allow any possibly-preempted writer to make progress
until the end of its writer serialization lock critical section.

Implement this lock-unlock technique for all seqcount_LOCKNAME_t with
an associated (PREEMPT_RT) sleeping lock.

References: 55f3560df975 ("seqlock: Extend seqcount API with associated locks")
Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20200519214547.352050-1-a.darwish@linutronix.de
---
include/linux/seqlock.h | 61 +++++++++++++++++++++++++++++++++-------
1 file changed, 51 insertions(+), 10 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index f3b7827..2bc9510 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -17,6 +17,7 @@
#include <linux/kcsan-checks.h>
#include <linux/lockdep.h>
#include <linux/mutex.h>
+#include <linux/ww_mutex.h>
#include <linux/preempt.h>
#include <linux/spinlock.h>

@@ -131,7 +132,23 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
* See Documentation/locking/seqlock.rst
*/

-#ifdef CONFIG_LOCKDEP
+/*
+ * For PREEMPT_RT, seqcount_LOCKNAME_t write side critical sections cannot
+ * disable preemption. It can lead to higher latencies, and the write side
+ * sections will not be able to acquire locks which become sleeping locks
+ * (e.g. spinlock_t).
+ *
+ * To remain preemptible while avoiding a possible livelock caused by the
+ * reader preempting the writer, use a different technique: let the reader
+ * detect if a seqcount_LOCKNAME_t writer is in progress. If that is the
+ * case, acquire then release the associated LOCKNAME writer serialization
+ * lock. This will allow any possibly-preempted writer to make progress
+ * until the end of its writer serialization lock critical section.
+ *
+ * This lock-unlock technique must be implemented for all of PREEMPT_RT
+ * sleeping locks. See Documentation/locking/locktypes.rst
+ */
+#if defined(CONFIG_LOCKDEP) || defined(CONFIG_PREEMPT_RT)
#define __SEQ_LOCK(expr) expr
#else
#define __SEQ_LOCK(expr)
@@ -162,10 +179,12 @@ static inline void seqcount_lockdep_reader_access(const seqcount_t *s)
*
* @lockname: "LOCKNAME" part of seqcount_LOCKNAME_t
* @locktype: LOCKNAME canonical C data type
- * @preemptible: preemptibility of above lockname
+ * @preemptible: preemptibility of above locktype
* @lockmember: argument for lockdep_assert_held()
+ * @lockbase: associated lock release function (prefix only)
+ * @lock_acquire: associated lock acquisition function (full call)
*/
-#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember) \
+#define SEQCOUNT_LOCKNAME(lockname, locktype, preemptible, lockmember, lockbase, lock_acquire) \
typedef struct seqcount_##lockname { \
seqcount_t seqcount; \
__SEQ_LOCK(locktype *lock); \
@@ -187,13 +206,33 @@ __seqprop_##lockname##_ptr(seqcount_##lockname##_t *s) \
static __always_inline unsigned \
__seqprop_##lockname##_sequence(const seqcount_##lockname##_t *s) \
{ \
- return READ_ONCE(s->seqcount.sequence); \
+ unsigned seq = READ_ONCE(s->seqcount.sequence); \
+ \
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
+ return seq; \
+ \
+ if (preemptible && unlikely(seq & 1)) { \
+ __SEQ_LOCK(lock_acquire); \
+ __SEQ_LOCK(lockbase##_unlock(s->lock)); \
+ \
+ /* \
+ * Re-read the sequence counter since the (possibly \
+ * preempted) writer made progress. \
+ */ \
+ seq = READ_ONCE(s->seqcount.sequence); \
+ } \
+ \
+ return seq; \
} \
\
static __always_inline bool \
__seqprop_##lockname##_preemptible(const seqcount_##lockname##_t *s) \
{ \
- return preemptible; \
+ if (!IS_ENABLED(CONFIG_PREEMPT_RT)) \
+ return preemptible; \
+ \
+ /* PREEMPT_RT relies on the above LOCK+UNLOCK */ \
+ return false; \
} \
\
static __always_inline void \
@@ -226,11 +265,13 @@ static inline void __seqprop_assert(const seqcount_t *s)
lockdep_assert_preemption_disabled();
}

-SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock)
-SEQCOUNT_LOCKNAME(spinlock, spinlock_t, false, s->lock)
-SEQCOUNT_LOCKNAME(rwlock, rwlock_t, false, s->lock)
-SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock)
-SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true, &s->lock->base)
+#define __SEQ_RT IS_ENABLED(CONFIG_PREEMPT_RT)
+
+SEQCOUNT_LOCKNAME(raw_spinlock, raw_spinlock_t, false, s->lock, raw_spin, raw_spin_lock(s->lock))
+SEQCOUNT_LOCKNAME(spinlock, spinlock_t, __SEQ_RT, s->lock, spin, spin_lock(s->lock))
+SEQCOUNT_LOCKNAME(rwlock, rwlock_t, __SEQ_RT, s->lock, read, read_lock(s->lock))
+SEQCOUNT_LOCKNAME(mutex, struct mutex, true, s->lock, mutex, mutex_lock(s->lock))
+SEQCOUNT_LOCKNAME(ww_mutex, struct ww_mutex, true, &s->lock->base, ww_mutex, ww_mutex_lock(s->lock, NULL))

/*
* SEQCNT_LOCKNAME_ZERO - static initializer for seqcount_LOCKNAME_t
\
 
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